1. Field of the Invention
The present invention relates to aqueous papercoating compositions comprising an alkali-swellable latex which is substantially insoluble in water.
2. Description of the Prior Art
The papercoating compositions generally known to this art are typically aqueous coating compositions containing a filler (pigment), a binder and, optionally, various adjuvants.
Such compositions are applied to paper for purposes of modifying the surface properties thereof, especially the brightness, the gloss, the printability, and the like.
The incorporation of a film-forming synthetic organic polymer latex as a binder material therefor, in the form of an aqueous emulsion, is a usual practice in the paper industry.
It is important, especially in order to control drying and to improve printability, that the water content of the coating composition and the water-soluble compounds contained in such water are not transferred rapidly from the coating composition into the paper substrate. To avoid these problems, a hydrocolloid or a natural thickening agent such as carboxymethyl cellulose or one of a synthetic variety, such as polyvinyl alcohol and alkali-soluble latices are typically included in the composition.
By the "water retention" of a papercoating composition is intended the ability of the composition to resist such water transfer.
This water retention must be high and well regulated because, otherwise, the centesimal composition of the coating composition varies very rapidly at the time of the coating, resulting in a complete modification of the rheological properties thereof, deteriorating its spreadability.
By "alkali-swellable latex" is intended a latex which is swollen by alkalis (in particular sodium hydroxide, potassium hydroxide, and aqueous ammonia) which are present in or added to the aqueous emulsion. These latices permit the formulation of coating compositions whose viscosities increase with a pH in the alkaline region.
The incorporation of alkali-swellable latices improves the water retention and the rheological properties of the coating composition.
Such insoluble alkali-swellable latices are described, for example, in FR-A-2,444,114 and comprise a styrene/butadiene/acrylic acid copolymer in which the viscosity of the coating bath is stabilized by addition of a polyalkylene glycol and in U.S. Pat. No. 3,793,244, where the acrylic acid is replaced with itaconic acid.
FR-A-2,006,324 describes an aqueous papercoating composition comprising a mixture of latices, including 80% to 99% by weight of an insoluble and non-alkali-swellable latex, and 1% to 20% by weight of an insoluble and alkali-swellable latex, rendered insoluble by means of 1,3-butadiene.
However, 1,3-butadiene is strongly hydrophobic, and this makes it necessary to employ large quantities of acidic monomers, which cause the polymer to go into solution; this results in a great increase in the viscosity of the bath and a deterioration in the binding capacity thereof.
Furthermore, this '324 patent is silent in respect of the latex particle size; likewise in respect of any requirement that the pigment must necessarily contain a minimum amount of CaCO.sub.3.